The homogeneous nucleation of ice in solution droplets is known to be an important mechanism of ice particle formation in the Earth's upper troposphere. It is known that the metastable cubic phase of ice can form when inorganic solution droplets freeze below about 200 K and that this may influence cloud properties and water vapor in the upper troposphere. However, many tropospheric aerosols contain a large and sometimes dominant proportion of oxygenated organic material in addition to inorganic substances. It is shown here that the threshold freezing temperature below which cubic ice forms and persists is shifted to considerably higher temperatures in solution droplets containing a carboxylic acid (2-hydroxypropane-1,2,3-tricarboxylic acid, citric acid, C6H8O7). Cubic and the stable hexagonal ice are the only phases to crystallize in micrometre sized citric acid droplets and therefore always exist in contact with aqueous citric acid solution. It is argued that the extremely high viscosity of low temperature aqueous citric acid solutions plays an important role in stabilizing cubic ice. The implications of these findings for ice clouds in the Earth's atmosphere are discussed.
